__name__上的抽象属性未强制执行(Abstract Property on __name__ not enforced)

 我们来看看生成的字节码： 
 
fun <T> getValue(): T {
    return 1000 as T
}

// becomes

public final getValue()Ljava/lang/Object;
   L0
    LINENUMBER 17 L0
    SIPUSH 1000
    INVOKESTATIC java/lang/Integer.valueOf (I)Ljava/lang/Integer;
    CHECKCAST java/lang/Object
    ARETURN
   L1
    LOCALVARIABLE this LSimpleClass; L0 L1 0
    MAXSTACK = 1
    MAXLOCALS = 1
 
 正如您所看到的，此方法不会将 1000转换为Long它只是确保对象的类型为java/lang/Object （嗯，它是）并返回1000作为Integer对象。 
 
 因此，您可以使用任何类型调用（注意：仅调用）此方法，这不会引发异常。 但是，将结果存储在变量中会调用实际强制转换，这可能会导致ClassCastException 
 
fun f3() {
    val simpleObject = SimpleClass()
    // L0
    //   LINENUMBER 16 L0
    //   NEW SimpleClass
    //   DUP
    //   INVOKESPECIAL SimpleClass.<init> ()V
    //   ASTORE 0

    simpleObject.getValue<SimpleClass>() // no problems
    // L1
    //   LINENUMBER 17 L1
    //   ALOAD 0
    //   INVOKEVIRTUAL SimpleClass.getValue ()Ljava/lang/Object;
    //   POP

    val number = simpleObject.getValue<SimpleClass>() // throws ClassCastException1
    // L2
    //   LINENUMBER 18 L2
    //   ALOAD 0
    //   INVOKEVIRTUAL SimpleClass.getValue ()Ljava/lang/Object;
    //   CHECKCAST SimpleClass
    //   ASTORE 1


    // L3
    //   LINENUMBER 19 L3
    //   RETURN
    // L4
    //   LOCALVARIABLE number LSimpleClass; L3 L4 1
    //   LOCALVARIABLE simpleObject LSimpleClass; L1 L4 0
    //   MAXSTACK = 2
    //   MAXLOCALS = 2
}
 
 但为什么将结果存储为Long? 抛出异常？ 再说一次，我们来看看字节码的差异： 
 
var number: Long? = null              |    var number: Long = 0
                                      |
      ACONST_NULL                     |        LCONST_0
      CHECKCAST java/lang/Long        |        LSTORE 0                
      ASTORE 0                        |

                number = simpleObject.getValue<Long>() [both]

      ALOAD 1                         |
              INVOKEVIRTUAL SimpleClass.getValue ()Ljava/lang/Object; [both]
      CHECKCAST java/lang/Long        |        CHECKCAST java/lang/Number
      ASTORE 0                        |        INVOKEVIRTUAL java/lang/Number.longValue ()J
                                      |        LSTORE 0
 
 如您所见， number: Long的字节码将函数结果转换为Number ，然后调用Number.longValue以将值转换为Long （Java中的long ） 
 
 但是，数字的字节码number: Long? 将函数结果直接转换为Long? （在Java中很Long ）导致ClassCastException 。 
 
 不确定，如果这种行为记录在某处。 但是， as运算符执行不安全的转换，编译器会警告它： 
 
Warning:(21, 16) Kotlin: Unchecked cast: kotlin.Int to T

Let's take a look into generated bytecode:
 
fun <T> getValue(): T {
    return 1000 as T
}

// becomes

public final getValue()Ljava/lang/Object;
   L0
    LINENUMBER 17 L0
    SIPUSH 1000
    INVOKESTATIC java/lang/Integer.valueOf (I)Ljava/lang/Integer;
    CHECKCAST java/lang/Object
    ARETURN
   L1
    LOCALVARIABLE this LSimpleClass; L0 L1 0
    MAXSTACK = 1
    MAXLOCALS = 1
 
As you can see, this method does not cast 1000 to a Long it simply ensures, that the object is of type java/lang/Object (well, it is) and returns the 1000 as an Integer object.
 
Therefore, you may call (note: call only) this method with any type and this will not throw exception. However, storing the result in a variable invokes real cast which may lead to ClassCastException
 
fun f3() {
    val simpleObject = SimpleClass()
    // L0
    //   LINENUMBER 16 L0
    //   NEW SimpleClass
    //   DUP
    //   INVOKESPECIAL SimpleClass.<init> ()V
    //   ASTORE 0

    simpleObject.getValue<SimpleClass>() // no problems
    // L1
    //   LINENUMBER 17 L1
    //   ALOAD 0
    //   INVOKEVIRTUAL SimpleClass.getValue ()Ljava/lang/Object;
    //   POP

    val number = simpleObject.getValue<SimpleClass>() // throws ClassCastException1
    // L2
    //   LINENUMBER 18 L2
    //   ALOAD 0
    //   INVOKEVIRTUAL SimpleClass.getValue ()Ljava/lang/Object;
    //   CHECKCAST SimpleClass
    //   ASTORE 1


    // L3
    //   LINENUMBER 19 L3
    //   RETURN
    // L4
    //   LOCALVARIABLE number LSimpleClass; L3 L4 1
    //   LOCALVARIABLE simpleObject LSimpleClass; L1 L4 0
    //   MAXSTACK = 2
    //   MAXLOCALS = 2
}
 
But why storing the result as a Long? throws an exception? Again, let's take a look at the differences in bytecode:
 
var number: Long? = null              |    var number: Long = 0
                                      |
      ACONST_NULL                     |        LCONST_0
      CHECKCAST java/lang/Long        |        LSTORE 0                
      ASTORE 0                        |

                number = simpleObject.getValue<Long>() [both]

      ALOAD 1                         |
              INVOKEVIRTUAL SimpleClass.getValue ()Ljava/lang/Object; [both]
      CHECKCAST java/lang/Long        |        CHECKCAST java/lang/Number
      ASTORE 0                        |        INVOKEVIRTUAL java/lang/Number.longValue ()J
                                      |        LSTORE 0
 
As you can see, the bytecode for number: Long casts the function result to a Number and then calls Number.longValue in order to convert the value to a Long (long in Java)
 
However, the bytecode for number: Long? casts the function result directly into the Long? (Long in Java) which leads to ClassCastException. 
 
Not sure, if this behavior documented somewhere. However, the as operator performs unsafe cast and the compiler warns about it:
 
Warning:(21, 16) Kotlin: Unchecked cast: kotlin.Int to T